Effective stubble management will be essential to control weeds and reduce the pressure on following crops this autumn. Glyphosate will also play a key role, but it’s essential to pay attention to detail and tailor the strategy to the situation.
Bayer’s Roger Bradbury urges farmers and spray operators to review how they use glyphosate to ensure they maximise performance in the current season and take appropriate stewardship steps to retain efficacy for the future.
While Glyphosate is highly effective and relatively easy to use, these strengths are also the biggest dangers because it’s easy to overlook the basics, says Roger.
“Sub-optimal performance means you aren’t getting the level of weed control you’ve paid for and expect. Potentially more serious is that poor practice could lead to the development of glyphosate resistance,” he explains.
Mode of action
Glyphosate is the sole HRAC Group 9 active – it inhibits an important enzyme for amino acid production, hence plant growth. It is systemic, so it is translocated to the roots and shoots. It needs active growth to work effectively.
Moderate temperatures (15–25OC) favour best efficacy. But even in cooler conditions (5 –15OC), glyphosate works well because there is still active growth. As it gets cooler, the metabolism slows, so symptoms just take longer to appear. For post-harvest stubble management and autumn stale seedbeds, you would not expect temperature to be an impediment to efficacy.
At other times of the year, there is more risk of high or low temperatures affecting performance. During periods of prolonged cold, uptake, translocation and final efficacy will be compromised because plant metabolism shuts down
Equally, high temperatures affect plant physiology which limits uptake and translocation. In fact, this is the case for plants under any type of stress, which is somewhat counterintuitive as you may think it is easier to control plants that are already weakened. Avoid spraying during the heat of the day, this could even be the case in early autumn if there is a heatwave similar to 2023.
Avoid application during the stem extension phase of growth, usually in late spring. The natural flow of sugars is upwards in support of the developing stem and flower but away from the roots, this can result in re-growth from the base and disappointing results.
“Lastly, there are better results from morning applications due to more drying time and plants are mobilising the glyphosate,” he adds.
Water quality and volume
Hard water and high pH typically occur together, but the combination is not significantly greater than the individual factors.
“Typically, for Roundup branded formulations, it is not usually technically necessary to add a water conditioner except in extreme situations. For other products with different formulations, it may be required,” explains Roger.
Roundup trials with buffering agents showed no improvement in performance for water between pH 4 – pH 8.
Hard water may have a pH of 7.5 – 8.5, but in trials, from pH 1 – 9, there was no difference to Roundup performance, because it always forms acid spray solutions of pH 4.5 – 5.
Harvested rainwater is the best option for diluting glyphosate. If a water conditioner is required, Mr Bradbury points out that it needs to be added to the spray tank first to remove the cations from the spray solution before adding the glyphosate.
Water volumes, higher than 250 litres/ha, make droplets containing a lower concentration of glyphosate. This is particularly true with lower rates of glyphosate and can lead to poorer results. In stubbles and stale seedbeds 100 litres/ha is sufficient, with the higher glyphosate concentrations per droplet improving uptake.
Formulation and rates
Since the ban on ethoxylated tallow amine (ETA) formulations in 2016, product choice has become more important for glyphosate efficacy. Many generic products use alkyl phosphate ester formulations which have more variable performance than ethoxylated tallow amine formulations, which were banned in 2016. Using a modern formulation like Roundup ensures the active reaches the target and provides more flexibility around timing and conditions.
Optimum rates depend on the weed spectrum and size, well-developed plants usually need a higher rate. Refer to rate tables on the product label and consult your agronomist.
“Don’t attempt to offset a lower rate by using an adjuvant,” advises Roger. “In trials, it hasn’t given reliable control, choose the right rate from the start.
Note larger well-established plants of all types generally need a higher rate, consult your agronomist.
Independent trials
Glyphosate needs to land and settle on the target to take effect. In general, stubbles and stale seedbeds contain small, recently emerged weeds that can be difficult to hit consistently.
Bayer trials in partnership with independent consultant, Tom Robinson, tested Roundup performance using four different nozzles:
- Lechler ID3 (90% drift reduction)
- Hypro GuardianAir (75% drift reduction)
- Hypro 3D (angled forward and back)
- 110 fan jet.
All were tested at 0.5m and 1m boom heights.
The fan jet increased drift with no improvement in control. The 3D gave the best control but is more prone to drift. The Lechler ID3 was the least drifty but with slightly lower weed control, the Hypro GuardianAir is a good compromise option.
“In ideal conditions, up to force 2, the 3D is the best option, at higher windspeeds use drift reducing nozzles. Angled nozzles like the 3D improve spray retention because more of the target is hit directly by the spray,” says Tom.
Anomalous result for the 3D at 1m due to temporary drop in wind speed.
Setting the boom lower, at 0.5m, compared with 1m reduces drift and improves application quality. Tom recommends 12km/h as a forward speed for herbicide applications, going faster can cause turbulence which affects the spray. Follow-up trials in 2023, however, found that drift reducing nozzles may be used up to 13.5km/h.
Cultivation and glyphosate
The Weed Resistance Action Group (WRAG) guidelines on preventing glyphosate resistance outline how to use cultivation to minimise resistance risks. It advises to avoid glyphosate applications to any weeds that have survived a previous spray. In practice, this means that you should always cultivate between glyphosate applications when managing fields in autumn.
Weed control based on glyphosate applications and a pre-em + post-em programme.
An ADAS study on stale seedbeds contributed to the WRAG guidelines, it found that two applications of glyphosate between harvest and drilling is the optimum for weed control without excessive workload, cost and resistance risk.
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